1. Field of the Invention
The present invention relates to an electric contact suitable for use in connector terminals for connecting electric circuits, for instance.
2. Description of the Prior Art
In electric contacts used for connector terminals, it is indispensable that the contact resistance is small and further stable without being subjected to the influence of mechanical friction, heat cycles, exposure to corrosive atmosphere, etc. Therefore, noble metals such as gold, silver, platinum, palladium, etc. excellent in abrasion resistance and corrosion resistance are widely used as the metallic material for electric contacts. However, when the electric contact is formed only of these noble metals, since the cost is high, it has been usual that the copper-based base material is electro-plated with a noble metal.
However, where the noble metal is directly plated on the copper-based base material, there exists a problem in that the contact resistance increases with the elapse of time, because atoms of the metallic base material are diffused into the plated noble metal.
To overcome this problem, conventionally a nickel layer is plated on the base material and then a noble metal layer is further plated on the nickel layer to prevent the atoms of the base material from being diffused into the noble metal layer.
In the noble metals used for the electric contacts, palladium-based metal such as palladium or palladium-nickel alloys are widely used, because the cost is low; the abrasion resistance is high; and the contact resistance is low. Therefore, where electric contacts are formed in accordance with the conventional way, a nickel layer with a thickness of 1 to 2 .mu.m is formed on a copper-based base material (substrate), for instance, and further a palladium-based layer is plated on the nickel layer. In this case, however, it has been well known that the durability of the electric contact, in particular the corrosive resistance thereof is seriously influenced by the thickness of the palladium-based layer formed by plating.
In practice, a 0.6 to 1 .mu.m thick palladium-based layer has been required. Further, where a higher reliability is required in particular, a 1 to 2 .mu.m thick palladium-based layer has been formed. In other words, it has been difficult to reduce the thickness of the costly palladium-based layer, thus increasing the cost thereof.